Ferromagnetic frame for magnetic resonance imaging

ABSTRACT

An apparatus for providing a B 0  magnetic field for a magnetic resonance imaging system. The apparatus includes at least one permanent B 0  magnet to contribute a magnetic field to the B 0  magnetic field for the MRI system and a ferromagnetic frame configured to capture and direct at least some of the magnetic field generated by the B 0  magnet. The ferromagnetic frame includes a first post having a first end and a second end, a first multi-pronged member coupled to the first end, and a second multi-pronged member coupled to the second end, wherein the first and second multi-pronged members support the at least one permanent B 0  magnet.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 62/946,000, titled“FERROMAGNETIC FRAME FOR MAGNETIC RESONANCE IMAGING,” filed on Dec. 10,2019, which is incorporated by reference in its entirety herein.

BACKGROUND

Magnetic resonance imaging (MRI) provides an important imaging modalityfor numerous applications and is widely utilized in clinical andresearch settings to produce images of the inside of the human body. Asa generality, MRI is based on detecting magnetic resonance (MR) signals,which are electromagnetic waves emitted by atoms in response to statechanges resulting from applied electromagnetic fields. For example,nuclear magnetic resonance (NMR) techniques involve detecting MR signalsemitted from the nuclei of excited atoms upon the re-alignment orrelaxation of the nuclear spin of atoms in an object being imaged (e.g.,atoms in the tissue of the human body). Detected MR signals may beprocessed to produce images, which in the context of medicalapplications, allows for the investigation of internal structures and/orbiological processes within the body for diagnostic, therapeutic and/orresearch purposes.

SUMMARY

Some embodiments are directed to an apparatus for providing a B₀magnetic field for a magnetic resonance imaging (MRI) system. Theapparatus comprises: at least one permanent B₀ magnet to contribute amagnetic field to the B₀ magnetic field for the MRI system; and aferromagnetic frame configured to capture and direct at least some ofthe magnetic field generated by the at least one permanent B₀ magnet.The frame comprises: a first post having a first end and a second end; afirst multi-pronged member coupled to the first end; and a secondmulti-pronged member coupled to the second end, wherein the first andsecond multi-pronged members support the at least one permanent B₀magnet.

Some embodiments are directed to a method, the method comprising:imaging a patient using a magnetic resonance imaging (MRI) system. TheMRI system comprises: at least one permanent B₀ magnet to contribute amagnetic field to a B₀ magnetic field for the MRI system; and aferromagnetic frame configured to capture and direct at least some ofthe magnetic field generated by the at least one permanent B₀ magnet.The ferromagnetic frame comprises: a first post having a first end and asecond end; a first multi-pronged member coupled to the first end; and asecond multi-pronged member coupled to the second end, wherein the firstand second multi-pronged members support the at least one permanent B₀magnet.

Some embodiments are directed to a frame for capturing and directing atleast some of a B₀ magnetic field generated by a magnetic resonanceimaging (MRI) system. The frame comprises a ferromagnetic frameconfigured to capture and direct at least some of the B₀ magnetic fieldgenerated by at least one permanent B₀ magnet. The ferromagnetic framecomprises: a first post having a first end and a second end; a firstmulti-pronged member coupled to the first end; and a secondmulti-pronged member coupled to the second end, wherein the first andsecond multi-pronged members support the at least one permanent B₀magnet.

Some embodiments are directed to an apparatus for providing a B₀magnetic field for a magnetic resonance imaging (MRI) system. Theapparatus comprises: at least one permanent B₀ magnet to contribute amagnetic field to the B₀ magnetic field for the MRI system; and aferromagnetic frame configured to capture and direct at least some ofthe magnetic field generated by the B₀ magnet. The frame comprises: afirst plate configured to support the at least one permanent B₀ magnet;and a first post attached to the first plate using a first connectionassembly, wherein the first connection assembly includes: a firstconnector that connects the first post and the first plate; and a secondconnector attached to the first connector.

Some embodiments are directed to a frame for capturing and directing atleast some of a B₀ magnetic field generated by a magnetic resonanceimaging (MRI) system. The frame comprises a ferromagnetic frameconfigured to capture and direct at least some of the B₀ magnetic fieldgenerated by at least one permanent B₀ magnet. The ferromagnetic framecomprises: a first post comprising a body portion, a first end, and asecond end, each of the first end and the second end comprising alayered junction coupled to the body portion of the first post; and afirst plate coupled to the first end of the first post, wherein thefirst plate supports the at least one permanent B₀ magnet.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIG. 1 illustrates exemplary components of an example magnetic resonanceimaging (MRI) system, in accordance with some embodiments of thetechnology described herein;

FIG. 2 illustrates an exemplary embodiment of an apparatus for providinga B₀ magnetic field for an MRI system, in accordance with someembodiments of the technology described herein;

FIG. 3 illustrates an exemplary embodiment of an apparatus for providinga B₀ magnetic field for an MRI system, the assembly including radialblades, in accordance with some embodiments of the technology describedherein;

FIGS. 4A-4C illustrate different embodiments of blades used as part ofthe example apparatus shown in FIG. 3, in accordance with someembodiments of the technology described herein;

FIG. 5 illustrates simulated gradient field decay over time fordifferent embodiments of an apparatus for providing a B₀ magnetic fieldfor an MRI system, in accordance with some embodiments of the technologydescribed herein;

FIGS. 6A-6C illustrate views of an apparatus for providing a B₀ magneticfield for an MRI system and including non-conductive support structures,in accordance with some embodiments of the technology described herein;

FIG. 7 illustrates a portable MRI system including the apparatus of FIG.3, in accordance with some embodiments of the technology describedherein;

FIG. 8 illustrates the use of the portable MRI system of FIG. 7 tocapture a magnetic resonance image of a patient's head, in accordancewith some embodiments of the technology described herein;

FIGS. 9A-9D illustrate views of an apparatus for providing a B₀ magneticfield for an MRI system, the assembly including ferromagnetic connectorsand blades, in accordance with some embodiments of the technologydescribed herein; and

FIGS. 10A-10B illustrate views of an apparatus for providing a B₀magnetic field for an MRI system, the assembly including ferromagneticplates and connection assemblies, in accordance with some embodiments ofthe technology described herein.

DETAILED DESCRIPTION

Conventional magnetic resonance imaging (MRI) systems are overwhelminglyhigh-field systems, particularly for medical or clinical MRIapplications. The general trend in medical imaging has been to produceMRI scanners with increasingly greater field strengths, with the vastmajority of clinical MRI scanners operating at 1.5 T or 3 T, with higherfield strengths of 7 T and 9 T used in research settings. As usedherein, “high-field” refers generally to MRI systems presently in use ina clinical setting and, more particularly, to MRI systems operating witha main magnetic field (i.e., a B₀ field) at or above 1.5 T, thoughclinical systems operating between 0.5 T and 1.5 T are often alsocharacterized as “high-field.” By contrast, “low-field” refers generallyto MRI systems operating with a B₀ field of less than or equal toapproximately 0.2 T, though systems having a B₀ field of between 0.2 Tand approximately 0.3 T have sometimes been characterized as low-fieldas a consequence of increased field strengths at the high end of thehigh-field regime.

Some low-field MRI systems increase accessibility to the imaging regionby employing a magnet assembly having a C-shaped design. Such a designuses a C-shaped steel frame to support the magnetic components of theMRI system, with a single post connecting two halves of the MRI systemwith the imaging region located therebetween. Examples of such C-shapeddesigns are described in U.S. Pat. No. 10,353,030, titled “Low-FieldMagnetic Resonance Imaging Methods and Apparatus”, granted on Jul. 16,2019, which is incorporated by reference herein in its entirety.However, the inventors have recognized that there are benefits (e.g., areduced system weight and/or increased field efficiency) to using adesign with additional supports.

Accordingly, the inventors have developed a lighter frame than aC-shaped design to support the B₀ magnets of an MRI system. Inparticular, the inventors have developed a forked frame, which reducesthe weight of the total system by reducing the amount of material (e.g.,steel) used to support the B₀ magnets as compared to a C-shaped design.In addition, the frame developed by the inventors reduces the conductionof eddy currents in the frame caused by the gradient fields, therebyincreasing gradient field efficiency of the MRI system and improvingimage quality by reducing eddy current-related artefacts.

Another benefit of the forked frame design developed by the inventors isthat the complexity of shimming may be reduced to achieve a desireddegree of homogeneity of the main magnet field. The asymmetry of aC-shaped design induces an asymmetry in the B₀ magnetic field, which maybe compensated by shimming to provide a suitably homogenous B₀ field inthe imaging region of the MRI system. The symmetry of the forked framedesign developed by the inventors may reduce the degree to whichshimming must be performed, thereby simplifying the manufacturingprocess for an MRI system, which makes it quicker and cheaper.

The inventors have further appreciated that attaching blades to theforked frame can enhance the main magnetic field and the gradientmagnetic fields of the MRI system. The blades can be sparsely arrangednear the gradient coils to provide improved gradient field efficiencyduring imaging. Additionally, positioning the blades near the B₀ magnetscan increase DC field efficiency by directing the magnetic flux towardthe imaging region.

The inventors have developed an apparatus for providing a B₀ magneticfield for an MRI system. In some embodiments the apparatus may includeat least one permanent B₀ magnet (e.g., a magnet comprising NdFeB, SmCo,AlNiCo, FeN, and/or other permanent magnet materials). The at least onepermanent B₀ magnet may produce a magnetic field to contribute to the B₀magnetic field. The apparatus may also include a ferromagnetic frameconfigured to capture and direct at least some of the magnetic fieldgenerated by the B₀ magnet. In some embodiments, the frame may include afirst post with a first end and a second end, a first multi-pronged(e.g., forked) member coupled to the first end, and a secondmulti-pronged member coupled to the second end. The first and secondmulti-pronged members may support the at least one permanent B₀ magnet.In some embodiments, the apparatus may also include a second post havinga third end and a fourth end. A third multi-pronged member may becoupled to the third end and a fourth multi-pronged member coupled tothe fourth end.

In some embodiments, the first post, first multi-pronged member, andsecond multi-pronged member may each be formed of ferromagnetic material(e.g., steel, silicon steel, etc.). In some embodiments, the secondpost, third multi-pronged member, and fourth multi-pronged member eachmay also be formed of ferromagnetic material. Forming components of theframe out of a ferromagnetic material may direct magnetic flux of the B₀magnet to increase field homogeneity and/or increase the B₀ magneticfield strength within the imaging region of the MRI system.

In some embodiments, the first multi-pronged member may include a stemand two prongs coupled to the stem. The two prongs may be spaced apartfrom one another by a gap. Each of the two prongs may be curved.

In some embodiments, the first multi-pronged member may be locatedopposite the third multi-pronged member. There may be a gap between thefirst and third multi-pronged member. In some embodiments, the secondmulti-pronged member may be located opposite the fourth multi-prongedmember. There may be a gap between the second and the fourthmulti-pronged member. The gaps may be air gaps, and may reduce eddycurrent conduction throughout the ferromagnetic frame.

In some embodiments, the at least one permanent B₀ magnet is a bi-planarmagnet and may include first concentric permanent magnet rings andsecond concentric permanent magnet rings. The first and thirdmulti-pronged members may support the first concentric permanent magnetrings, and the second and fourth multi-pronged members support thesecond concentric permanent magnet rings.

In some embodiments, the first and third multi-pronged members may becoupled to a first non-conductive component (e.g., a plastic component,a fiberglass component, etc.), and the first concentric permanent magnetrings may be arranged on a surface of the first non-conductivecomponent. The second and fourth multi-pronged members may be coupled toa second non-conductive component, and the second concentric permanentmagnet rings may be arranged on a surface of the second non-conductivecomponent. The first non-conductive component and the secondnon-conductive component may be substantially planar.

In some embodiments, the apparatus may also include a first plurality offerromagnetic blades (e.g., manufactured from steel, silicon steel,etc.). The first plurality of ferromagnetic blades may be coupled to thefirst multi-pronged member or the third multi-pronged member at an endof each of the first plurality of ferromagnetic blades. The end of eachof the first plurality of ferromagnetic blades may be placed within aslot formed within the first multi-pronged member or the thirdmulti-pronged member. In some embodiments, the ferromagnetic blades inthe first plurality of ferromagnetic blades are arranged to extendradially from a common center. The blades may not contact the commoncenter.

In some embodiments, each of the ferromagnetic blades may have aconstant height and/or width along its length. Alternatively, in someembodiments, the height and/or width of each of the first plurality offerromagnetic blades may vary along its length. For example, in someembodiments, the height and/or width of a ferromagnetic blade may betapered. In some embodiments, the first plurality of blades includes atleast 16 blades and at most 24 blades. Alternatively, in someembodiments, the first plurality of blades includes between 8 and 32blades.

In some embodiments, the apparatus may include a second plurality offerromagnetic blades. The second plurality of ferromagnetic blades maybe coupled to the second multi-pronged member or the fourthmulti-pronged member at an end of each of the second plurality offerromagnetic blades. The end of each of the second plurality offerromagnetic blades may be placed within a slot formed within thesecond multi-pronged member or the fourth multi-pronged member.

In some embodiments, the first post and the second post may be arrangedat an angle of 180° such that the at least one permanent B₀ magnet islocated between the first post and the second post. Alternatively, thefirst post and the second post may be arranged at angle in a range from100° to 180°. The first post and the second post may be arranged at anangle in a range from 120° to 145°. The first post and the second postmay be arranged at an angle of 120°. Reducing the angle between thefirst post and the second post may provide more accessibility to oneside of the imaging region while maintaining most of the advantages ofthe symmetric configuration of 180°.

The inventors have further developed a low-field MRI system. In someembodiments, the system includes an apparatus for providing a B₀magnetic field for the MRI system. The apparatus may include at leastone permanent B₀ magnet to produce a magnetic field to contribute to theB₀ magnetic field. The apparatus may also include a ferromagnetic frameconfigured to capture and direct at least some of the magnetic fieldgenerated by the B₀ magnet. In some embodiments, the frame may include afirst post with a first end and a second end, a first multi-prongedmember (e.g., a forked member) coupled to the first end, and a secondmulti-pronged member coupled to the second end. The first and secondmulti-pronged members may support the at least one permanent B₀ magnet.In some embodiments the MRI system may include gradient coils configuredto generate magnetic fields to provide spatial encoding of magneticresonance (MR) signals, a radio frequency transmit coil, and a powersystem. The power system may be configured to provide power to thegradient coils and/or the radio frequency transmit coil. In someembodiments, the MRI system may be used to capture at least one MRimage.

In some embodiments, a low-field MRI system comprising a ferromagneticframe with one or more multi-pronged members, in accordance withembodiments described herein, may be used for imaging a patient.

The inventors have appreciated that lighter-weight, less complex, andbetter-performing forked frame designs may be attained by attaching someor all of the ferromagnetic components to non-ferromagnetic components.Thus, the inventors have developed frame designs in which ferromagneticblades (e.g., 2 to 8 blades) are attached to a substantially planarnon-ferromagnetic component rather than being fitted into slots machinedin the forked frame. In this way, assembly and manufacture may besimplified. Additionally, the ferromagnetic blades may be positionedparallel to one of the x- or y-gradient magnetic fields, rather thanradially, which improves gradient field efficiency.

In some embodiments, such forked-frame designs may include connectorsextending between and securing the first post to the second post in adirection perpendicular to the ferromagnetic blades, thus improvingstructural rigidity and providing additional gradient efficiency to thex- and/or y-gradient magnetic field. For example, in some embodimentsthe connectors comprise ferromagnetic bars extending between the firstpost and the second post. The ferromagnetic blades are positioned toextend along a direction substantially perpendicular to the length ofthe ferromagnetic bars.

In some embodiments, the ferromagnetic bars are formed as a first and asecond bar. The first and the second bar are positioned substantiallyparallel to one another. In some embodiments, a portion of the first baris separated from a portion of the second bar by a gap. The gap has awidth that is less than a quarter of the spacing between the firstmulti-pronged member and the third multi-pronged member. Alternativelyor additionally, the gap has a width in a range from 75 mm to 100 mm.

The inventors have appreciated that manufacturing complexity of theframe may be reduced by forming the ferromagnetic frame out of multiple,layered components. For example, if the frame is constructed as a singlepiece, the machining of the piece may be more complex than if the frameis constructed from a number of smaller components (e.g., that may becut from sheet metal stock or smaller blocks of material). Additionally,insulation layers may be inserted between the components of the frame toreduce eddy current circulation during operation of the resulting MRIsystem.

Accordingly, the inventors have developed an apparatus for providing aB₀ magnetic field for a magnetic resonance imaging (MRI) system, theapparatus including at least one permanent B₀ magnet to contribute amagnetic field to the B₀ magnetic field for the MRI system and aferromagnetic frame configured to capture and direct at least some ofthe magnetic field generated by the B₀ magnet. The frame includes afirst plate configured to support the at least one permanent B₀ magnetand a first post attached to the first plate using a first connectionassembly. The first connection assembly includes a first connector thatconnects the first post and the first plate to one another, and a secondconnector attached to the first connector. In some embodiments, thesecond connector may be configured to provide additional enhancement ofmagnetic flux within the apparatus.

In some embodiments, the ferromagnetic frame also includes a second postattached to the first plate using a second connection assembly. Thesecond connection assembly includes a third connector that connects thesecond post and the first plate, and a fourth connector attached to thethird connector.

In some embodiments, the ferromagnetic frame also includes a secondplate disposed opposite the first plate and configured to support the atleast one permanent B₀ magnet. The second plate is attached to the firstpost using a third connection assembly and to the second post using afourth connection assembly. The third connection assembly includes afifth connector that connects the first post to the second plate and asixth connector attached to the fifth connector. The fourth connectionassembly includes a seventh connector that connects the second post tothe second plate and an eighth connector attached to the seventhconnector.

In some embodiments, the first connector comprises a ferromagneticplate. For example, in some embodiments, the first connector comprisessilicon steel. In some embodiments, the first connect is secured to thefirst post and the first plate using multiple fasteners.

In some embodiments, the second connector is secured to the first postby multiple fasteners. The multiple fasteners may pass through the firstconnector and enter the first post to secure the second connector to thefirst post.

In some embodiments, the ferromagnetic frame also includes at least onepermanent magnet coupled to an interior face of the first post toprovide additional and/or alternative homogenization of the B₀ magneticfield. The at least one permanent magnet, in some embodiments, comprisesa cylindrical permanent magnet.

In some embodiments, the at least one permanent magnet comprises a firstpermanent magnet and a second permanent magnet. The first permanentmagnet and the second permanent magnet are disposed along the length ofthe first post. In some embodiments, the first permanent magnet has afirst polarization and the second permanent magnet has a secondpolarization opposite the first polarization. For example, in someembodiments, one of the first and second polarizations is directedtoward the interior face of the first post.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, ferromagnetic frames for MRI. It shouldbe appreciated that various aspects described herein may be implementedin any of numerous ways. Examples of specific implementations areprovided herein for illustrative purposes only. In addition, the variousaspects described in the embodiments below may be used alone or in anycombination, and are not limited to the combinations explicitlydescribed herein.

FIG. 1 is a block diagram of components of a MRI system 100. In theillustrative example of FIG. 1, MRI system 100 comprises computingdevice 104, controller 106, pulse sequences store 108, power managementsystem 110, and magnetics components 120. It should be appreciated thatsystem 100 is illustrative and that an MRI system may have one or moreother components of any suitable type in addition to or instead of thecomponents illustrated in FIG. 1. However, a MRI system will generallyinclude these high level components, though the implementation of thesecomponents for a particular MRI system may differ.

As illustrated in FIG. 1, magnetics components 120 comprise B₀ magnet122, shim coils 124, RF transmit and receive coils 126, and gradientcoils 128. Magnet 122 may be used to generate the main magnetic fieldB₀. Magnet 122 may be any suitable type or combination of magneticscomponents that can generate a desired main magnetic B₀ field. In someembodiments, magnet 122 may be a permanent magnet, an electromagnet, asuperconducting magnet, or a hybrid magnet comprising one or morepermanent magnets and one or more electromagnets and/or one or moresuperconducting magnets. In some embodiments, magnet 122 may be abi-planar permanent magnet and, in some embodiments, ma include multiplesets of concentric permanent magnet rings.

Gradient coils 128 may be arranged to provide gradient fields and, forexample, may be arranged to generate gradients in the B₀ field in threesubstantially orthogonal directions (X, Y, Z). Gradient coils 128 may beconfigured to encode emitted MR signals by systematically varying the B₀field (the B₀ field generated by magnet 122 and/or shim coils 124) toencode the spatial location of received MR signals as a function offrequency or phase. For example, gradient coils 128 may be configured tovary frequency or phase as a linear function of spatial location along aparticular direction, although more complex spatial encoding profilesmay also be provided by using nonlinear gradient coils.

MRI is performed by exciting and detecting emitted MR signals usingtransmit and receive coils, respectively (often referred to as radiofrequency (RF) coils). Transmit/receive coils may include separate coilsfor transmitting and receiving, multiple coils for transmitting and/orreceiving, or the same coils for transmitting and receiving. Thus, atransmit/receive component may include one or more coils fortransmitting, one or more coils for receiving and/or one or more coilsfor transmitting and receiving. Transmit/receive coils are also oftenreferred to as Tx/Rx or Tx/Rx coils to generically refer to the variousconfigurations for the transmit and receive magnetics component of anMRI system. These terms are used interchangeably herein. In FIG. 1, RFtransmit and receive coils 126 comprise one or more transmit coils thatmay be used to generate RF pulses to induce an oscillating magneticfield B₁. The transmit coil(s) may be configured to generate anysuitable types of RF pulses.

Power management system 110 includes electronics to provide operatingpower to one or more components of the low-field MRI system 100. Forexample, power management system 110 may include one or more powersupplies, gradient power components, transmit coil components, and/orany other suitable power electronics needed to provide suitableoperating power to energize and operate components of MRI system 100. Asillustrated in FIG. 1, power management system 110 comprises powersupply 112, power component(s) 114, transmit/receive switch 116, andthermal management components 118 (e.g., cryogenic cooling equipment forsuperconducting magnets). Power supply 112 includes electronics toprovide operating power to magnetic components 120 of the MRI system100. For example, power supply 112 may include electronics to provideoperating power to one or more B₀ coils (e.g., B₀ magnet 122) to producethe main magnetic field for the low-field MRI system. Transmit/receiveswitch 116 may be used to select whether RF transmit coils or RF receivecoils are being operated.

Power component(s) 114 may include one or more RF receive (Rx)pre-amplifiers that amplify MR signals detected by one or more RFreceive coils (e.g., coils 126), one or more RF transmit (Tx) powercomponents configured to provide power to one or more RF transmit coils(e.g., coils 126), one or more gradient power components configured toprovide power to one or more gradient coils (e.g., gradient coils 128),and one or more shim power components configured to provide power to oneor more shim coils (e.g., shim coils 124).

As illustrated in FIG. 1, MRI system 100 includes controller 106 (alsoreferred to as a console) having control electronics to sendinstructions to and receive information from power management system110. Controller 106 may be configured to implement one or more pulsesequences, which are used to determine the instructions sent to powermanagement system 110 to operate the magnetic components 120 in adesired sequence (e.g., parameters for operating the RF transmit andreceive coils 126, parameters for operating gradient coils 128, etc.).As illustrated in FIG. 1, controller 106 also interacts with computingdevice 104 programmed to process received MR data. For example,computing device 104 may process received MR data to generate one ormore MR images using any suitable image reconstruction process(es).Controller 106 may provide information about one or more pulse sequencesto computing device 104 for the processing of data by the computingdevice. For example, controller 106 may provide information about one ormore pulse sequences to computing device 104 and the computing devicemay perform an image reconstruction process based, at least in part, onthe provided information.

FIG. 2 depicts a schematic of an apparatus 200 for providing a B₀magnetic field for an MRI system, in accordance with some embodiments ofthe technology described herein. The apparatus 200 may include B₀magnets 210 and a frame 220 that captures electromagnetic flux producedby the B₀ magnet 210 and transfers the flux to the opposing permanentmagnet to increase the flux density between B₀ magnets 210. The B₀magnets 210 may be arranged in a bi-planar geometry and may eachinclude, for example, a plurality of concentric permanent magnet rings210 a-d as depicted in FIG. 2. In particular, as visible in FIG. 2, B₀magnet 210 comprises a lower portion having a first set of concentricpermanent magnet rings including: an inner ring of permanent magnets 210a, a first middle ring of permanent magnets 210 b, a second middle ringof permanent magnets 210 c, and an outer ring of permanent magnets 210d. The upper portion of B₀ magnet 210 includes another set of concentricpermanent magnet rings. In other embodiments, the B₀ magnets 210 may,additionally or alternatively, include electromagnets, superconductingmagnets, other permanent magnets, or any suitable combinations thereof.

The permanent magnet material used may be selected depending on thedesign requirements of the system. For example, according to someembodiments, the permanent magnets (or some portion thereof) may be madeof NdFeB, which produces a magnetic field with a relatively highmagnetic field per unit volume of material once magnetized. According tosome embodiments, SmCo material is used to form the permanent magnets,or some portion thereof. While NdFeB produces higher field strengths(and in general is less expensive than SmCo), SmCo exhibits less thermaldrift and thus provides a more stable magnetic field in the face oftemperature fluctuations. Other types of permanent magnet material(s)may be used as well, as the aspects of the technology described hereinare not limited in this respect. In general, the type or types ofpermanent magnet material utilized will depend, at least in part, on thefield strength, temperature stability, weight, cost and/or ease of userequirements of a given B₀ magnet implementation.

The permanent magnet rings 210 a-d may be sized and arranged to producea homogenous field of a desired strength in the central region (e.g.,the field of view and/or the imaging region) between permanent magnets210. It may be appreciated that B₀ magnet 210 may include any suitablenumber of permanent magnet rings, not only four permanent magnet ringsas depicted in FIG. 2. In some embodiments, such as pictured inexemplary FIG. 2, the permanent magnet rings 210 a-d may be formed of aplurality of permanent magnet blocks. The dimensions (e.g., height,width) of the blocks may be varied to facilitate the production of amagnetic field of desired strength and homogeneity. For example, in someembodiments, the heights of the permanent magnet rings may increase awayfrom the center of the magnet. For instance, in some embodiments, theheight of permanent magnet ring 210 b may be larger than that ofpermanent magnet 210 a, the height of permanent magnet ring 210 c may belarger than that of permanent magnet 210 b, and the height of permanentmagnet 210 d may be larger than that of permanent magnet 210 d. Aspectsof varying heights and/or widths of concentric permanent magnet ringsare further described in U.S. Pat. Pub. No.: 2019/0353726, titled “B0Magnet Methods and Apparatus For A Magnetic Resonance Imaging System”,filed on May 20, 2019, which is incorporated by reference herein in itsentirety.

The apparatus 200 further includes frame 220 configured to capturemagnetic flux generated by B₀ magnets 210 and direct it to the opposingside of the B₀ magnet to increase the magnetic flux density in betweenB₀ magnets 210, thereby increasing the field strength within the fieldof view of the B₀ magnet. By capturing magnetic flux and directing it tothe region between B₀ magnets 210, less permanent magnet material can beused in B₀ magnets 210 to achieve a desired field strength, thusreducing the size, weight, and cost of the B₀ magnet. Alternatively, forgiven permanent magnets, the field strength can be increased, thusimproving the signal-to-noise ratio (SNR) of the system without havingto use increased amounts of permanent magnet material.

For exemplary apparatus 200, frame 220 includes a first post 222 a and asecond post 222 b, the first post 222 a having a first end 223 a and asecond end 223 b and the second post 22 b having a third end 223 c and afourth end 223 d. Multi-pronged members 224 a, 224 b are coupled to thefirst end 223 a and second end 223 b of first post 222 a andmulti-pronged members 224 c, 224 d are coupled to the third end 223 cand fourth end 223 d of the second post 222 b. Multi-pronged members 224a-d may be coupled to the first post 222 a and second post 222 b throughstem members 226.

As shown in FIG. 2, first and second posts 222 a,b may be positionedopposite each other at an angle of 180° such that B₀ magnets 210 arepositioned between the first and second posts 222 a,b. However, it maybe appreciated that first and second posts 222 a,b may be positioned atangles other than 180° (e.g., 120°) in order to increase accessibilityto one side of the field of view of B₀ magnets 210 while retaining mostof the advantages of the symmetry of the apparatus 200. For example, insome embodiments, first and second posts 222 a,b may be positioned atany angle in a range from 100° to 180° (e.g., 120°, 135°, 150°, 165°, or180°. Alternatively, in other embodiments, first and second posts 222a,b may be positioned at any angle in a range from 120° to 145°.

Multi-pronged members 224 a-d may capture electromagnetic flux generatedby B₀ magnets 210 and direct the electromagnetic flux to first andsecond posts 222 a,b to be circulated via a magnetic return path of theframe. This electromagnetic flux capturing may increase the flux densityin the field of view of the B₀ magnet, in accordance with someembodiments of the technology described herein. As shown in FIG. 2,multi-pronged members 224 a-d includes two prongs with a collector area229 disposed between the two prongs. However, in other embodiments,multi-pronged members 224 a-d may include two or more prongs (e.g., 4prongs, 6 prongs) to occupy the collector area 229 and increase captureof electromagnetic flux generated by B₀ magnets 210.

In some embodiments, frame 220, including multi-pronged members 224 a-dand first and second posts 222 a,b, may be constructed of any desiredferromagnetic material, for example, low carbon steel and/or CoFe,and/or silicon steel, etc. to provide the desired magnetic propertiesfor the frame 220. In some embodiments, first and second posts 222 a,band/or multi-pronged members 224 a-d may be constructed of laminatedferromagnetic material (e.g., any of the aforementioned ferromagneticmaterials) in order to reduce persistent circulation of eddy currentsaround the cross-section of the multi-pronged members 224 a-d. In suchembodiments, first and second posts 222 a,b and/or multi-pronged members224 a-d may be formed of laminations disposed in planes substantiallyorthogonal to the planes of B₀ magnets 210.

In some embodiments, multi-pronged members 224 a-d may attach tonon-conductive components (not pictured in FIG. 2) configured to supportB₀ magnets 210, as described herein including with reference to FIGS.6A-6C. Multi-pronged members 224 a-d may be generally designed to reducethe amount of magnetic material needed to support the permanent magnetswhile still providing sufficient cross-section for the return path forthe magnetic flux generated by B₀ magnets 210. Such a design may be asmuch as 20% lighter than a C-shaped frame (e.g., weighing betweenapproximately 220 kg and 300 kg for a structure with an accessible gapof 35 cm and providing a B₀ magnetic field of 65 mT and a shimmedhomogeneity of 500 ppm peak-to-peak over a 20 cm diameter sphere).

Additionally, because multi-pronged members 224 a-d reduce the amount ofmagnetic material used in frame 220, multi-pronged members 224 a-d alsoreduce the surface area available for eddy current conduction duringoperation of the gradient coils of the MRI system. This reduction mayresult in reduced time constants and increased overall gradient fieldefficiency of the MRI system.

Further, in some embodiments, multi-pronged members 224 a-d may bearranged with a gap 228 between ends of opposing multi-pronged members224 a-d, as seen in exemplary FIG. 2. The gap 228 may be an air gap.Such an air gap may eliminate a conduction path across the directionbetween opposing posts 222. The gap 228 may accordingly further reduceeddy current conduction in the frame 220 during MR imaging.

As depicted in the example of FIG. 2, a collector area 229 (e.g., anopen area over the B₀ magnets 210) may be located between multi-prongedmembers 224 a-d. The collector area 229 may channel electromagnetic fluxfrom the magnet rings into the first and second posts 222 a,b. Thecollector area 229 may also provide enhancement to the gradient field bythe inclusion of a number of conductive blades 340 as depicted in theexample of FIG. 3.

FIG. 3 illustrates an embodiment of an apparatus 300 having blades 340configured to enhance gradient magnetic fields generated by an MRIsystem that includes apparatus 330, in accordance with some embodimentsof the technology described herein. Blades 340 may be arranged to coverthe surface behind the gradient coils (not pictured) in a sparse manner,providing improved gradient field efficiency while minimizing eddycurrent conduction. The apparatus 300 may include a number of blades340, such as 16 or 24 blades, to provide the improved gradient fieldefficiency. Alternatively, the apparatus 300 may include a number ofblades 340 in a range from 16 to 24 blades, or in a range from 8 to 32blades.

In some embodiments, the blades 340 may be arranged in a radial manner,extending towards a common center in the collection area 229 betweenmulti-pronged members 224 a-d. Blades 340 may not meet or touch thecommon center in order to prevent the formation of a conduction path foreddy currents between opposing blades 340. As a result, the eddy currenttime constants for exemplary apparatus 300 may be less than half theeddy current time constants for comparable C-shaped designs. Althoughnot shown in FIG. 3, in some embodiments, the blades 340 may be coupledto one or more non-conductive elements at the common center of thecollection area 229 for stability (e.g., the ends closer to the centermay slide into plastic slots, as shown by non-conductive element 660 ofFIG. 6A).

While blades 340 may increase the weight of apparatus 300, blades 340also increase the collection of electromagnetic flux from B₀ magnets210, thereby increasing DC-field efficiency, in accordance with someembodiments of the technology described herein. The increased DC-fieldefficiency provided by blades 340 may allow for a reduction in theweight of the permanent magnet material used in B₀ magnets 210,potentially reducing the overall raw materials cost of the apparatus300.

In some embodiments, to provide improved gradient field efficiency,blades 340 may be formed of a ferromagnetic material. The blades may beformed of, for example, low carbon steel, CoFe, and/or silicon steel toprovide the desired magnetic properties. The blades 340 may be formed ofa same ferromagnetic material as frame 220, or may be formed of adifferent ferromagnetic material as frame 220.

In some embodiments, blades 340 may be formed separately from themulti-pronged members 224 a-d. The blades may be coupled tomulti-pronged members 224 a-d by, for example, being inserted intomachined slots (not pictured) in multi-pronged members 224 a-d. In suchembodiments, the blades 340 may be formed by, for example, stamping orlaser cutting from sheet metal stock. Alternatively, blades 340 andmulti-pronged members 224 a-d may be cast together as a single piece toreduce the number of parts needed for assembly.

In some embodiments, the blades 340 may have specific dimensions inorder to provide the desired magnetic properties. For example, theblades 340 may be tall enough to avoid magnetic saturation by the B₀magnets 210 in order to provide enhancement to the gradient coils.However, the blades 340 may not be too tall, or they will provideadditional surface area for eddy current conduction. For example, theblades 340 may be approximately half the height of the multi-prongedmembers 224 a-d in order to provide these desired magnetic properties.Similarly, blades 340 may be thin in order to reduce eddy currentconduction caused by the additional material. For example, in someembodiments the blades 340 may be approximately 5 mm in width.Alternatively, the blades 340 may be made thicker while the total numberof blades 340 may be reduced in order to reduce assembly complexity.

In some embodiments, blades 340 may have constant profiles along theirlengths. In other embodiments, blades 340 may have tapered profilesalong their lengths. Examples of such blade profiles are depicted inFIGS. 4A-4C. FIG. 4A shows an example of a blade 340 having a constantheight H and a constant width W along its length, in accordance withsome embodiments of the technology described herein.

FIG. 4B shows an example of a blade 340 having a tapered width, inaccordance with some embodiments of the technology described herein. Thewidth of blade 340 changes along the blade's length from a first widthW1 at a first end to a second, smaller width W2 at a second end of theblade 340. The first end of the blade may be the end attached tomulti-pronged members 224 a-d of the apparatus 300 such that the secondend is positioned near the common center of the collection area.

FIG. 4C shows an alternative example of a blade 340 having a taperedheight, in accordance with some embodiments of the technology describedherein. The height of blade 340 may change along the blade's length froma first height H1 at a first end to a second, smaller height H2 at asecond end of the blade 340. The first end of the blade may be attachedto multi-pronged members 224 a-d of the apparatus 300 such that thesecond end is positioned near the common center of the collection area.

FIG. 5 depicts simulated gradient field decay over time for differentconfigurations of an apparatus for providing a B₀ magnetic field for anMRI system, in accordance with some embodiments of the technologydescribed herein. At time equal to zero, a simulated gradient fieldalong a single axis is quickly ramped down. The gradient field thendecays at different rates depending on the structure near the gradientcoils. The three curves, from top to bottom, show gradient field decayover time for an apparatus including solid plates supporting the B₀magnets, an apparatus including blades (e.g., blades 340) arranged nearthe B₀ magnets, and an apparatus including empty space near the B₀magnets (e.g., collection area 229 between prongs of multi-prongedmembers 224 a-d, as described in FIG. 2). The blade configuration offersa faster gradient field decay than a more conventional solid plateconfiguration. The empty space configuration generates 12 mT/m at fullcurrent while the blade configuration generations 14.7 mT/m at fullcurrent. The solid plate provides 15.6 mT/m.

FIGS. 6A-6C depict different views of an apparatus 600 for providing aB₀ magnetic field for an MRI system, in accordance with some embodimentsof the technology described herein. Apparatus 600 is similar toapparatus 300 of FIG. 3, but includes non-conductive support structures650 a and 650 b above and below multi-pronged members 224 a-d. Apparatus600 also includes non-conductive element 660 coupled with central endsof blades 340. Non-conductive support structures 650 a and 650 b and/ornon-conductive element 660 may be made of any suitable non-conductivematerial, including but not limited to plastic and/or fiberglass.

In some embodiments, non-conductive support structures 650 a and 650 bmay be grooved to provide locations and support to the blades 340.Additionally, the non-conductive support structures 650 a and 650 b mayprotect the blades 340 from environmental damage (e.g., dust, dirt). Thenon-conductive support structure 650 b may support the B₀ magnets 210,which may be mounted directly to a surfaces of the non-conductivesupport structure 650 b. In some embodiments, such surfaces may besubstantially planar.

Using the techniques described herein, the inventors have developedportable, low power MRI systems capable of being brought to the patient,providing affordable and widely deployable MRI where it is needed. FIG.7 shows an example of a portable, low-field MRI system 700 including theapparatus 300 of FIG. 3, in accordance with some embodiments of thetechnology described herein. The apparatus 300 may be supported by abase 710. Base 710 may house the power components and/or electronicsdiscussed in connection with FIG. 1, including power componentsconfigured to operate the MRI system 700.

Base 710 may also include one or more transport mechanisms 720 whichenable point-of-care use of MRI system 700, in accordance with someembodiments of the technology described herein. In the example of FIG.7, the transport mechanisms 720 are depicted as wheels, but othertransport mechanisms may be used. In some embodiments, transportmechanisms 720 may include a motorized component 725 may be provided toallow the MRI system 700 to be driven from location to location, forexample, using a control such as a joystick or other control mechanismprovided on or remote from the MRI system 700. In this manner, MRIsystem 700 can be transported to the patient and maneuvered to thebedside to perform imaging, as illustrated in FIG. 8.

FIG. 8 depicts the use of the portable MRI system of FIG. 7 to perform abrain scan of a patient, in accordance with some embodiments of thetechnology described herein. During the brain scan, the MRI system 700may be used to capture at least one magnetic resonance image of thepatient for clinical use.

As described herein, in some embodiments, the blades may be arranged ina radial manner. However, in some embodiments, an alternativearrangement may be employed. One example of such alternative arrangementis shown in FIGS. 9A-9D, which depict different views of an apparatus900 for providing a B₀ magnetic field for an MRI system, the assemblyincluding ferromagnetic connectors and blades, in accordance with someembodiments of the technology described herein. FIG. 9A shows theapparatus 900 in a partially disassembled state, and FIG. 9B shows a topview of the apparatus 900 in a partially disassembled state. FIGS. 9Cand 9D show the apparatus 900 assembled with laminate panels 940 andshims 950 and in an exploded view, respectively.

For example, in some embodiments and as shown in FIGS. 9A and 9B,apparatus 900 includes B₀ magnets 910 and a frame that captureselectromagnetic flux produced by the B₀ magnet 910 and transfers theflux to the opposing permanent magnet to increase the flux densitybetween B₀ magnets 910. The B₀ magnets 910 are arranged in a bi-planargeometry and may each include, for example, a plurality of concentricpermanent magnet rings as described in connection with the embodiment ofFIG. 2.

In some embodiments, the frame includes posts 922 coupled tomulti-pronged members 924, as described in connection with the exampleof FIG. 2. The frame also includes one or more connectors 925 extendingbetween opposite ends of posts 922. The connectors 925 may secure theposts 922 to one another. The connectors 925 may be positioned betweenthe multi-pronged members 924, in some embodiments.

In some embodiments, the connectors 925 may be elongated bars extendingbetween the posts 922. The bars may have intermediate portions with areduced thickness relative to end portions of the bars, as shown in FIG.9B. In some embodiments, the frame may include multiple connectors 925(e.g., two, two or more, etc.), and the connectors 925 may be positionedsubstantially parallel one another. The connectors 925 may also bepositioned substantially parallel to a direction of one of the gradientfields (e.g., one of the X- or Y-gradient fields) to enhance thegenerated gradient field strength during MR imaging.

In some embodiments, there may be a gap G1 between the intermediateportions of the connectors 925. The gap G1 may be approximately 80 mm inwidth or may be in a range from 75 mm to 100 mm in width. Alternatively,in some embodiments, the width of gap G1 may be determined relative to alength of the posts 922 (e.g., a distance between opposing multi-prongedmembers 924 of the frame). For example, the width of gap G1 may be lessthan a quarter of the length of the posts 922. Limiting the width of gapG1 and the thickness of connectors 925 may reduce the magnitude of eddycurrents circulating through the frame during MR imaging.

In some embodiments, the frame, including posts 922, multi-prongedmembers 924, and connectors 925, may be constructed of any desiredferromagnetic material, for example, low carbon steel and/or CoFe,and/or silicon steel, etc. to provide the desired magnetic propertiesfor the frame. In some embodiments, posts 922, multi-pronged members924, and/or connectors 925 may be constructed of laminated ferromagneticmaterial (e.g., any of the aforementioned ferromagnetic materials) inorder to reduce persistent circulation of eddy currents around thecross-section of the multi-pronged members 924 and/or connectors 925. Insuch embodiments, first and second posts 922, multi-pronged members 924,and/or connectors 925 may be formed of laminations disposed in planessubstantially orthogonal to the planes in which the rings of B₀ magnets910 are positioned.

In some embodiments, apparatus 900 may include blades 926. Blades 926may be similar to blades 340 of apparatus 300, as described inconnection with FIG. 3. Blades 926, however, may be arrangedsubstantially parallel to a direction of one of the gradient fields(e.g., one of the X- or Y-gradient fields) rather than in a radialarrangement as in apparatus 300. Blades 926 may be arrangedsubstantially parallel to a direction of one of the gradient fields toprovide improved gradient field efficiency during operation of the MRIsystem. The apparatus 900 may include a number of blades 926, such as 4or 6 blades, to provide the improved gradient field efficiency.Alternatively, the apparatus 900 may include a number of blades 926 in arange from 2 to 8 blades. In the examples of FIGS. 9A and 9B, fourblades 926 are shown, positioned in pairs on either side of theconnectors 925. The blades 926 of each pair may be separated by a gap G2having a width of approximately 127.4 mm. In some embodiments, the gapG2 may have a width in a range from 110 mm to 140 mm.

In some embodiments, to provide improved gradient field efficiency,blades 926 may be formed of a ferromagnetic material. The blades may beformed of, for example, low carbon steel, CoFe, and/or silicon steel toprovide the desired magnetic properties. The blades 926 may be formed ofa same ferromagnetic material as the other components of the frame ormay be formed of a different ferromagnetic material as the frame.

In some embodiments, blades 926 may be formed separately from themulti-pronged members 924 and/or connectors 925. In such embodiments,the blades 926 may be formed by, for example, stamping or laser cuttingfrom sheet metal stock. Alternatively, blades 926, multi-pronged members924, and connectors 925 may be cast together as a single piece to reducethe number of parts needed for assembly.

In some embodiments, blades 926 may have constant profiles along theirlengths, similar to the example of blade 340 depicted in FIG. 4A. FIG.4A shows an example of a blade having a constant height H and a constantwidth W along its length. In some embodiments, blades 926 may have aconstant width of approximately 7.5 mm or a constant width in a rangefrom 5 mm to 10 mm. In some embodiments, blades 926 may have a constantheight of approximately 66 mm or a constant height in a range from 50 mmto 100 mm. In some embodiments, blades 926 may have a constant length ofapproximately 190 mm or in a range from 170 mm to 210 mm.

In some embodiments, blades 926, multi-pronged members 924, and/orconnectors 925 may attach to non-conductive components configured tosupport the apparatus 900 and B₀ magnets 910. For example, eachmulti-pronged member 924 may be secured to and spaced apart from anopposing multi-pronged member 924 by spacers 928. In some embodiments,spacers 928 may be formed of plastic or any other suitablenon-conductive material. Additionally, spacers 928 may be configured toprovide rigidity non-conductive supports 930.

In some embodiments, the apparatus may include one or morenon-conductive supports 930 configured to cover the components of theframe and provide support to B₀ magnets 910 and blades 926. In someembodiments, structural foam may be inserted into the spaces between thenon-conductive supports 930, multi-pronged members 924, connectors 925,and/or blades 926. The non-conductive supports 930 may be formed of anon-conductive laminate material such as G-10. The non-conductivesupports 930 may be positioned on an outward-facing surface of themulti-pronged members 924 and/or between an inward-facing surface of themulti-pronged members 924 and the B₀ magnets 910, where “inward-facing”indicates facing towards the region between the B₀ magnets 910. Becausethe blades 926 are secured to non-conductive supports 930 rather thanmulti-pronged members 924, no slots in multi-pronged members 924 may bemachined, reducing manufacturing complexity of the apparatus 900relative to the apparatus 300 and/or 600.

In some embodiments, the non-conductive supports 930 may be fastened tothe multi-pronged members 924 and connectors 925 by fasteners 932, insome embodiments. The fasteners may extend through the multi-prongedmembers 924 and/or connectors 925 into posts 922. Additionally, in someembodiments, blades 926 may be fastened to non-conductive supports 930by additional fasteners (not shown) extending through the non-conductivesupports 930 into blades 926.

In some embodiments, apparatus 900 may include laminate panels 940and/or shims 950, as shown in the examples of FIGS. 9C and 9D. Laminatepanels 940 may be positioned on inward-facing surfaces of B₀ magnets910, and shims 950 may be placed on laminate panels 940 thereafter. Insome embodiments, laminate panels 940 may include at least oneconductive layer patterned to form one or more gradient coils, or aportion of one or more gradient coils, capable of producing orcontributing to magnetic fields suitable for providing spatial encodingof detected MR signals when operated in a low-field MRI apparatus. Insome embodiments, the laminate panel may comprise one or more conductivelayers patterned to form one or more X-gradient coils (or portionsthereof), one or more Y-gradient coils (or portions thereof) and/or oneor more Z-gradient coils (or portions thereof).

Alternatively or additionally, laminate panels 940 may include at leastone conductive layer patterned to form one or more transmit and/orreceive coils, or a portion of one or more transmit and/or receivecoils, configured to stimulate an MR response by producing a B₁excitation field (transmit) and/or receive an emitted MR signal(receive) when operated in conjunction with magnetic componentsconfigured to produce a B₀ field and/or corresponding gradient fieldsfor spatially encoding received MR signals. Furthermore, in someembodiments, the laminate panels 940 may include additional magneticcomponents such as one or more shim coils arranged to generate magneticfields in support of the system to, for example, increase the strengthand/or homogeneity of the B0 field, counteract deleterious field effectssuch as those created by operation of the gradient coils, loadingeffects of the object being imaged, or to otherwise support themagnetics of the low field MRI system. Additional details regarding thefabrication and structure of such laminate panels is provided in U.S.Pat. No. 10,495,712 filed on Sep. 29, 2017 and titled “Low FieldMagnetic Resonance Imaging Methods and Assembly,” which is incorporatedherein by reference in its entirety.

In some embodiments, shims 950 may be positioned adjacent laminatepanels 940 and configured to improve homogeneity and provide correctionto the field strength of the B₀ magnetic field within the imaging regionof the MRI system. For example, passive pieces of ferromagnetic material(e.g., steel) may be positioned to adjust the B₀ magnetic field profileof the MRI system. Shims 950, for example, may be formed as sheets ofmagnetic material that have been magnetized in a desired pattern toproduce a magnetic field to improve the profile of the B0 magneticfield. Shims 950 are shown in the example of FIG. 9B as including twosheets of magnetic material (lower sheets) and a plastic retainer (top)configured to locate the orientation of and secure the shims to thelaminate panels 940. It should be appreciated that shims 950 may includefewer or more than two sheets of magnetic material in some embodiments.Additional details regarding the fabrication and structure of magneticsheet shims is provided in U.S. Pat. No. 10,613,168 filed Mar. 22, 2017and titled “Methods and Apparatus for Magnetic Field Shimming,” which isincorporated by reference herein in its entirety.

FIGS. 10A-10B illustrate views of an apparatus 1000 for providing a B₀magnetic field for an MRI system, in accordance with some embodiments ofthe technology described herein. In some embodiments, the apparatus 1000includes posts 1022 secured to plates 1030 by connection assemblies1024. The plates 1030 may be configured to support the B₀ magnets 1010.

In some embodiments, the connection assemblies 1024 may include a firstconnector 1024 a and a second connector 1024 b. The first connector 1024a may connect one of the posts 1022 to one of the plates 1030. Forexample, and as shown in FIG. 10B, the first connector 1024 a may be asubstantially planar plate extending over the plate 1030 so thatfasteners 1025 a may extend through the first connector 1024 a andsecure the first connector 1024 a to the plate 1030. First connector1024 a may be secured to the post 1022 by additional fasteners 1025 bextending through the second connector 1024 b, the first connector 1024a, and the post 1022. Forming the connection assembly 1024 out ofmultiple “layered” components may reduce manufacturing costs (e.g., bysimplifying machining processes). In some embodiments, insulatingcomponents (not shown) may be inserted between components of theconnection assembly 1024 and/or between the connection assembly 1024 andthe post 1022 to reduce or mitigate eddy current circulation in theapparatus 1000.

In some embodiments, the second connector 1024 b may be configured toincrease the magnetic flux capacity of the apparatus 1000. For example,the second connector 1024 b may have a wedge-like shape as shown in theexamples of FIGS. 10A and 10B to direct and concentrate magnetic fluxfrom the posts 1022 back into the imaging region between the B₀ magnets1010.

In some embodiments, plates 1030 may be configured to support B₀ magnets1010. Plates 1030 may be formed from solid ferromagnetic sheet material.In some embodiments, plates 1030 may include one or more holes to reducethe weight of the plates 1030 and/or to allow for cooling or venting ofthe apparatus 1000 during MR imaging.

In some embodiments, posts 1022, connection assemblies 1024, and plates1030 may be constructed of any desired ferromagnetic material, forexample, low carbon steel and/or CoFe, and/or silicon steel, etc. toprovide the desired magnetic properties for the apparatus 1000. In someembodiments, posts 1022, connection assemblies 1024, and/or plates 1030may be constructed of laminated ferromagnetic material (e.g., any of theaforementioned ferromagnetic materials) in order to reduce persistentcirculation of eddy currents around the cross-section of the connectionassemblies 1024 or plates 1030. In such embodiments, posts 1022,connection assemblies 1024, and/or plates 1030 may be formed oflaminations disposed in planes substantially orthogonal to the planes ofB₀ magnets 1010.

In some embodiments, apparatus 1000 may include additional permanentmagnets 1026 positioned on inward-facing surfaces of posts 1022. Thepermanent magnets 1026 may be positioned and/or shaped to reduceinhomogeneity of the B₀ magnetic field and may be used in addition to oras a replacement for shim coils and/or passive shims positioned adjacentthe B₀ magnets 1010. For example, the permanent magnets 1026 may becylindrical or elliptical in cross-sectional shape. The permanentmagnets 1026 may be disposed along the length of posts 1022.

In some embodiments, permanent magnets 1026 may be polarized along adirection perpendicular to a plane of the inward-facing surfaces of theposts 1022 (e.g., toward or away from a common center of the concentricB₀ permanent magnet rings 1010). In some embodiments having twopermanent magnets 1026, each of the two permanent magnets 1026 may haveopposing polarizations. For example, a first of the permanent magnets1026 may have a polarization directed toward the inward-facing surfacesof the posts 1022 and a second of the permanent magnets 1026 may have apolarization direction away from the inward-facing surfaces of the posts1022.

It should be appreciated that while the examples of FIGS. 10A and 10Bshow two permanent magnets 1026 attached to each post 1022, additionalpermanent magnets 1026 or fewer permanent magnets 1026 may be used insome embodiments. It should also be appreciated that permanent magnets1026 may be included in any of the embodiments described herein,including apparatuses 200, 300, 600, 900, and/or 1000.

Having thus described several aspects of at least one embodiment of thistechnology, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Various aspects of the technology described herein may be used alone, incombination, or in a variety of arrangements not specifically describedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

The terms “approximately,” “substantially,” and “about” may be used tomean within ±20% of a target value in some embodiments, within ±10% of atarget value in some embodiments, within ±5% of a target value in someembodiments, within ±2% of a target value in some embodiments. The terms“approximately,” “substantially,” and “about” may include the targetvalue.

What is claimed is:
 1. An apparatus for providing a B₀ magnetic fieldfor a magnetic resonance imaging (MRI) system, the apparatus comprising:at least one permanent B₀ magnet to contribute a magnetic field to theB₀ magnetic field for the MRI system; and a ferromagnetic frameconfigured to capture and direct at least some of the magnetic fieldgenerated by the at least one permanent B₀ magnet, the frame comprising:a first post having a first end and a second end; a first multi-prongedmember coupled to the first end; a second multi-pronged member coupledto the second end; a second post having a third end and a fourth end; athird multi-pronged member coupled to the third end; and a fourthmulti-pronged member coupled to the fourth end, wherein the first andsecond multi-pronged members support the at least one permanent B₀magnet.
 2. The apparatus of claim 1, wherein the first post and thesecond post are secured to one another by at least one ferromagneticconnector.
 3. The apparatus of claim 2, wherein the at least oneferromagnetic connector comprises a first bar and a second bar, whereinthe first bar is positioned substantially parallel to the second bar andwherein a portion of the first bar is separated from a portion of thesecond bar by a gap.
 4. The apparatus of claim 2, wherein the at leastone ferromagnetic connector further comprises at least one blade havinga length extending along a direction substantially perpendicular to alength of the at least one ferromagnetic connector.
 5. The apparatus ofclaim 1, wherein each of the first post, first multi-pronged member,second multi-pronged member, second post, third multi-pronged member,and fourth multi-pronged member comprises ferromagnetic material.
 6. Theapparatus of claim 1, wherein the first multi-pronged member includes astem and two prongs coupled to the stem, wherein the two prongs arespaced apart from one another by a gap and each of the two prongs iscurved.
 7. The apparatus of claim 1, wherein: the first multi-prongedmember is disposed opposite the third multi-pronged member with a gapbetween the first and third multi-pronged member; and the secondmulti-pronged member is disposed opposite the fourth multi-prongedmember with a gap between the second and the fourth multi-prongedmember.
 8. The apparatus of claim 1, wherein the at least one permanentB₀ magnet is a bi-planar magnet comprising first concentric permanentmagnet rings and second concentric permanent magnet rings, the first andthird multi-pronged members support the first concentric permanentmagnet rings, and the second and fourth multi-pronged members supportthe second concentric permanent magnet rings.
 9. The apparatus of claim1, further comprising a first plurality of ferromagnetic blades and asecond plurality of ferromagnetic blades, wherein: an end of each of thefirst plurality of ferromagnetic blades is coupled to the firstmulti-pronged member or the third multi-pronged member, an end of eachof the second plurality of ferromagnetic blades is coupled to the secondmulti-pronged member or the fourth multi-pronged member, andferromagnetic blades of the first and second pluralities offerromagnetic blades are arranged to extend radially from a commoncenter.
 10. A magnetic resonance imaging system, comprising: theapparatus of claim 1; a plurality of gradient coils configured to, whenoperated, generate magnetic fields to provide spatial encoding ofemitted magnetic resonance signals; at least one radio frequencytransmit coil; and a power system configured to provide power to theplurality of gradient coils and the at least one radio frequencytransmit coil.
 11. A method, comprising: imaging a patient using amagnetic resonance imaging (MRI) system, the MRI system comprising: atleast one permanent B₀ magnet to contribute a magnetic field to a B₀magnetic field for the MRI system; and a ferromagnetic frame configuredto capture and direct at least some of the magnetic field generated bythe at least one permanent Bo magnet, the ferromagnetic framecomprising: a first post having a first end and a second end; a firstmulti-pronged member coupled to the first end; a second multi-prongedmember coupled to the second end; a second post having a third end and afourth end; a third multi-pronged member coupled to the third end; and afourth multi-pronged member coupled to the fourth end, wherein the firstand second multi-pronged members support the at least one permanent B₀magnet.
 12. A frame for capturing and directing at least some of a B₀magnetic field generated by a magnetic resonance imaging (MRI) system,the frame comprising: a ferromagnetic frame configured to capture anddirect at least some of the B₀ magnetic field generated by at least onepermanent B₀ magnet, the ferromagnetic frame comprising: a first posthaving a first end and a second end; a first multi-pronged membercoupled to the first end; a second multi-pronged member coupled to thesecond end; a second post having a third end and a fourth end; a thirdmulti-pronged member coupled to the third end; and a fourthmulti-pronged member coupled to the fourth end, wherein the first andsecond multi-pronged members support the at least one permanent B₀magnet.
 13. The frame of claim 12, further comprising at least onespacer securing the first multi-pronged member to the thirdmulti-pronged member and securing the second multi-pronged member to thefourth multi-pronged member.
 14. The frame of claim 12, furthercomprising a first plurality of ferromagnetic blades.
 15. The frame ofclaim 14, wherein each of the first plurality of ferromagnetic blades isarranged to extend along a direction substantially parallel to one of anx- or y-gradient magnetic field.
 16. The frame of claim 14, wherein thefirst post and the second post are secured to one another by at leastone bar, and wherein each of the first plurality of ferromagnetic bladesis arranged to extend along a direction substantially perpendicular to alength of the at least one bar.
 17. The frame of claim 12, furthercomprising at least one first permanent magnet coupled to an interiorface of the first post and at least one second permanent magnet coupledto an interior face of the second post.
 18. The frame of claim 17,wherein: the at least one first permanent magnet comprises a firstpermanent magnet having a first polarization directed toward theinterior face of the first post and a second permanent magnet having asecond polarization opposite the first polarization, and the at leastone second permanent magnet comprises a third permanent magnet having athird polarization directed toward the interior face of the second postand a fourth permanent magnet having a fourth polarization opposite thethird polarization.